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When cosmic rays, especially high-energy galactic cosmic rays, strike Earth's atmosphere, they often produce a cascade of secondary sub-atomic particles called an "air shower". This diagram depicts an incoming cosmic ray (in red, at the top) and the resulting air shower which includes protons (green), neutrons (orange), pions (yellow), muons (purple), photons (blue), and electrons & positrons (pink). An actual air shower may consist of millions of particles, depending on the energy of the initial cosmic ray.
Click on image for full size Windows to the Universe original artwork by Randy Russell using a photo courtesy UCAR (Nicole Gordon).

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Cosmic Rays

Cosmic rays are a type of radiation
that comes from space. Cosmic rays aren't really "rays"; they are
subatomic particles (mostly protons)
with very high energies. Cosmic rays come from various places, including the
Sun, supernova
explosions, and extremely distant sources such as radio galaxies and quasars.
Because of their high energy, this type of particle
radiation can be dangerous to people and to equipment, though here on Earth
we are mostly shielded from them by our planet's magnetic
field and atmosphere.

Cosmic rays were discovered by the Austrian-American physicist Victor Hess
in 1912. Hess won the 1936 Nobel Prize in Physics for this discovery. The American
physicist Robert Andrews Millikan confirmed Hess discovery in 1925, and coined
the term "cosmic rays" for this new type of radiation from space.

There are several different types of cosmic rays, and the radiation comes from
various sources. Closest to home are solar cosmic rays, produced on the Sun
by solar flares and similar
energetic events. Solar cosmic
rays have lower energies (up to about 1010 electron volts per particle)
than do other types of cosmic rays. Galactic cosmic rays have higher energies
(roughly 1010 to 1015 eV) and are believed to come from
supernova explosions, black holes, and neutron
stars within our own Milky Way galaxy.
Even more energetic (1015 eV or higher) are the rare extragalactic
cosmic rays. Astronomers believe these particles come from beyond our galaxy,
but are otherwise unsure of their exact origins. They may come from the nuclei
of active galaxies, from quasars, or be produced during collisions between galaxies.
They might even be left over from exotic particle decay processes that occurred
when the Universe was young. A fourth, and also somewhat mysterious, type of
cosmic ray is called the anomalous cosmic rays (ACR). ACRs have unexpectedly
low energy levels, and may be produced at the edge of the heliosphere,
the boundary between the region where the Sun's magnetic
field is dominant and interstellar space.

What kinds of particles make up cosmic rays? Most, about 90%, are protons;
in other words, the ionized nuclei of normal hydrogen, the most abundant form
of matter in the Universe. Helium nuclei (also known as alpha particles), which
include 2 protons and 2 neutrons, account for about 9% of cosmic rays. Most
of the remaining 1% are electrons.
The nuclei of a broad range of other types of atoms
(including carbon, oxygen, iron, calcium, lithium, beryllium, boron, gallium
and others) are represented in small but measurable amounts in the cosmic ray
menagerie. Different types of cosmic rays (solar vs. galactic, for instance)
have slightly different abundances of different constituents.

Because of their high energies, cosmic rays can be dangerous to living creatures
and to machines (especially those that rely on electronics) alike. Fortunately
for us, Earth protects us from most cosmic rays. Because they are electrically
charged, cosmic rays interact with Earth's magnetic field. Earth's magnetism
deflects away many cosmic rays, and steers much of the remainder towards our
planet's sparsely populated polar regions. Most particles that make it past
Earth's magnetic shields collide with gases in our atmosphere. This is both
good and bad for those of us on the planet's surface. When cosmic rays collide
with atmospheric gases, the collisions generate new and different particles.
In general, these collisions dilute the energy of the particles, producing larger
numbers of less energetic particles. A single high-energy cosmic ray can produce
a shower of thousands or millions of secondary particles, as secondary particles
produced by collisions crash into other gas atoms and molecules, producing yet
more particles, and so on. Though this particle shower or cascade (technically
known as "spallation") generally produces lower energy, and thus less
lethal, particles, some of the secondary byproducts are actually more harmful
to living creatures.

How dangerous is cosmic ray radiation? Typically, normal Earthlings are exposed
to roughly 2.3 millisieverts (a measure of radiation dosage, abbreviated mSv)
of radiation per year. About 0.2 mSv of this dose, or about 9%, is due to cosmic
rays. In other words, if you stay on Earth, you normally have little to fear
from cosmic rays. However, if you decide to travel into space, and especially
if you venture beyond the protective bubble of Earth's magnetosphere,
cosmic rays could become a major problem. Scientists estimate that unshielded
humans in interplanetary space might receive 400 to 900 mSv of radiation per
year, mostly from cosmic rays. Astronauts on a 30 month long Mars mission could
be exposed to 1,000 mSv or more of radiation. This is comparable to the recommended
career limit of 1 to 4 Sieverts (1,000 to 4,000 mSv) of radiation exposure
for astronauts in low Earth orbit. Because cosmic rays have such high energies,
they can penetrate deeply into tissue and cause extensive damage to cellular
DNA, producing cancers and similar maladies.

The number of cosmic ray particles reaching Earth varies over time. Solar
activity varies dramatically over the course of an 11-year
cycle. Oddly, the danger from cosmic rays is least when the Sun is most
active and producing solar flares
and other dramatic "space weather
storms". The Sun's activity causes the heliosphere,
the vast region in space dominated by the Sun's magnetic
field, further outward. The heliosphere acts as another layer of magnetic
shielding from galactic and extragalactic cosmic rays; so when the Sun is active
this shield expands and fewer external cosmic rays reach Earth. When the Sun
is active, we are exposed to larger doses of solar cosmic rays; however, we
receive lesser doses of the higher energy, and thus more dangerous, galactic
and extragalactic cosmic rays. The net effect is less total danger from radiation
exposure. For this reason, when Earthlings do eventually venture beyond our
planet on missions to Mars and other interplanetary voyages, they will probably
do so when the Sun is at its most active phase!

Cosmic rays help produce some interesting natural phenomena as they zip through
our atmosphere. Some cosmic ray collisions with nitrogen
(the most abundant gas in Earth's atmosphere) transform the nitrogen atoms into
atoms of carbon-14 (or 14C for short). This radioactiveisotope of
carbon is absorbed by living creatures, and is the basis for carbon-14 dating;
you've probably heard of archeological artifacts and similar ancient remains
being dated using the carbon-14 method. Cosmic rays in the atmosphere may also
help to trigger lightning;
the ionized trails the particles leave behind in the air as they pass through
may be what's needed to allow atmospheric voltage differences to set off lightning
strikes. Finally, cosmic rays might play a role in cloud
formation; the radiation may help produce some cloud condensation nuclei, the
tiny particles that act as "seeds" that start water droplet formation
in clouds. These last two effects (triggering lightning and helping to seed
clouds) are somewhat speculative; scientists are still studying both phenomena.

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